Semiconductor power devices, including a MOSFET power device and an IGBT power device, which employ silicon (hereinafter referred to sometimes as “Si”) for the semiconductor material thereof, are devices typically used in inverters and apparatuses for electric power control. However, since the Si semiconductor properties have already reached the extremes of improvements thereof, the Si power devices exhibit almost the limit properties due to the physical properties of the semiconductor Si.
In contrast, since silicon carbide (hereinafter referred to as “SiC”) exhibits excellent physical properties as described below, SiC (4H—SiC) is more advantageous than Si especially for the power device material. The electric field strength that causes a dielectric breakdown in SiC is 10 times as high as the electric field strength that causes a dielectric breakdown in Si. The band gap in SiC is 2.9 times as wide as the band gap in Si. The thermal conductivity in SiC is 3.2 times as high as the thermal conductivity in Si. The temperature, at which SiC becomes an intrinsic semiconductor, is 3 to 4 times as high as the temperature, at which Si becomes an intrinsic semiconductor. Since the power device that employs semiconductor SiC is expected to exhibit not only a high breakdown voltage but also low on-resistance, developments of many SiC power devices have been vigorously explored recently. SiC diodes and such SiC rectifying devices, as well as SiC transistors, SiC thyristors, and such SiC switching devices, have been fabricated experimentally. Among the SiC switching devices, a trench-type insulated-gate field-effect transistor (hereinafter referred to as a “UMOSFET”) is attracting much attention especially as a device that facilitates further reduction of the resistance in the ON-state thereof, since the UMOSFET facilitates to increase the channel density by minimizing any of the trench gate structure thereof and the unit pattern thereof including a channel.
The SiC UMOSFET is manufactured in almost the same manner as a typical Si semiconductor device. Specifically, the SiC UMOSFET is manufactured in the form of an SiC semiconductor device of a trench-MOS-type by forming trenches in an SiC semiconductor substrate (hereinafter simply referred to sometimes as an “SiC substrate”) by anisotropic etching, by removing the oxide film used for an etching mask, by forming gate insulator films, by filling the trenches with polycrystalline silicon that will work as gate electrodes, and by forming a source electrode and a drain electrode.
The SiC substrate is hard physically and stable chemically. Therefore, the SiC substrate is tough to etch. Therefore, the reactive ion etching (hereinafter referred to as the “RIE”) used usually for forming trenches in a Si substrate is not employable for the mass-productive trench formation in the SiC substrate. For the mass-productive trench formation in the SiC substrate, there exists no way but to employ the physical etching (hereinafter referred to as the “dry etching”) that bombards the SiC substrate with accelerated plasma ions to etch the surface thereof. As compared with the trench shape control performed by the RIE in the Si substrate, it is not so easy for the dry etching to form trenches having a good profile in the SiC substrate. For example, it is quite hard to form the trench bottom having a U-shape, which is preferable for the breakdown voltage characteristics of the semiconductor device, only using dry etching. It is also quite hard to provide the trench side wall with excellent smoothness only using dry etching. As illustrated in FIG. 6, which shows a cross-sectional view of a trench 2 of about 3 μm in thickness seen obliquely, an edge corner 10 in the opening 3 of the trench 2 is sharp. Protrusions, surface unevenness and such defective shapes are created on the side wall 11 and the bottom 12 of the trench 2. The electric field is liable to localize to the defective shapes formed in the trench 2 as described above, causing a low dielectric breakdown voltage.
The above-described defective shapes and such problems, generated in the process of forming the trenches via physical dry etching that bombards the SiC substrate with plasma particles accelerated at a high frequency under the reduced pressure, can be avoided by thermally treating the trench inner surface in a mixed gas atmosphere containing hydrogen gas (hereinafter referred to as “H2 gas” or simply as “H2”) or argon gas (hereinafter referred to as “Ar gas” or simply as “Ar”) at 1700° C. or lower. Alternatively, the above-described defective shapes and such problems may be avoided by etching the trench inner surface with H2 at 1300° C. or higher under reduced pressure. The techniques described above for obviating the defective shapes and such problems are disclosed in Unexamined Japanese Patent Application Publication Nos. 2005-328013 and 2005-328014 (hereafter References 1 and 2).
It is known to the skilled persons in the art that smooth trench inner surfaces can be obtained in the Si semiconductor device by surface diffusing Si atoms, namely by annealing the Si semiconductor substrate with trenches formed therein in H2. It is quite possible to control the inner surface shape and inner surface properties of the trench in the SiC semiconductor substrate by surface diffusion of the Si atoms. See Unexamined Japanese Patent Application Publication No. 2003-229479.
References 1 and 2 disclose methods for obviating the defective shapes generated in forming the trenches by employing H2 at a high temperature. In the SiC substrate, not only silicon atoms but also carbon atoms exist in the trench inner surface as the composition elements. The carbon atoms are hazardous for the surface diffusion of Si atoms. Since it is hard to vigorously surface diffuse Si atoms, which is effective for smoothening the Si surface, in the SiC surface, the surface diffusion of Si atoms is not so effective to smoothen the SiC surface.
It has been found that the SiC surface can be etched more vigorously with high temperature H2 than with the surface diffusion of the composition atoms in the SiC substrate. Nonetheless, it has been found that the etching of the SiC surface with high-temperature H2 is hardly more controllable than the surface diffusion of the composition atoms in the SiC substrate. It has been found also that the high temperature treatment with H2 is more influential on the trench shape control. Since the trench shape will be changed excessively if the treatment with high temperature H2 is conducted without any modification, it is hard to employ the high temperature treatment with H2 as a practical manufacturing method for improving the trench shape.
Accordingly, there still remains a need for a manufacturing method for smoothening the inner surface of a trench and for shaping or rounding the opening edge corner and the bottom corner of the trench to prevent the electric field from localizing, in forming the trench in an SiC substrate by dry etching. The present invention addresses this need.